Comparison of Etomidate, Benzocaine, and MS222 Anesthesia with and without Subsequent Flunixin Meglumine Analgesia in African Clawed Frogs (Xenopus Laevis)

Abstract

Often few alternative anesthetics for exotic species are available, due to the small numbers of these animals used in research. In this study, we evaluated the depth and duration of anesthesia in Xenopus laevis after their immersion in 3 doses of etomidate (15, 22.5, and 30 mg/L) and in 3 doses of benzocaine (0.1%, 0.5%, and 1%) compared with the ‘gold standard,’ tricaine methanesulfonate (MS222; 2 g/L). We then chose an optimal dose for each alternative anesthetic according to induction time, duration of surgical plane, and time to complete recovery. The optimal etomidate and benzocaine doses (22.5 mg/L and 0.1%, respectively) as well as the MS222 dose were then used to achieve a surgical plane of anesthesia, with the addition of flunixin meglumine (25 or 50 mg/kg) administered in the dorsal lymph sac at the completion of mock oocyte harvest. Efficacy of the analgesic was assessed at 1, 3, 6, and 24 h postoperatively by using acetic acid testing (AAT). Histology of the liver, kidney, and tissues surrounding the dorsal lymph sac was performed at day 3, 14, and 28 in each group of animals.

Mild to moderate myocyte degeneration and necrosis were present in tissues surrounding the dorsal lymph sac at both flunixin meglumine doses after etomidate and benzocaine anesthesia. In addition, the 50-mg/kg dose of flunixin meglumine resulted in the death of 5 of the 12 frogs within 24 h, despite an otherwise uneventful anesthetic recovery. In conclusion, benzocaine and etomidate offer alternative anesthetic regimens, according to typical requirements for an anesthetic event. Flunixin meglumine at the 25-mg/ kg dose provided analgesic relief at the latest time point during etomidate dosage and at all time points during benzocaine dosage, but further characterization is warranted regarding long-term or repeated analgesic administration.